1. Field of the Invention
This invention relates generally to the field of signal synchronizing apparatus and more specifically to apparatus for synchronizing internally generated signals to an input signal in a manner which facilitates extraction of information from the inputted signal or from other signals locked to the inputted signal or which facilitates the generating of a signal containing information, which signal is synchronized to the inputted signal.
2. Description of the Prior Art
Present day techniques used to synchronize signals within a system, such as a video signal genlock device, clock recovery device, or the like, to externally generated signals rely on the stability of the externally generated signal which acts as an input reference signal and on the uninterrupted presence of the reference signal and on the uninterrupted presence of the reference signal to maintain a known relationship between the reference and internally generated signals.
One known technique assumes that the externally generated input reference signal will be very stable with changes in its characteristics occurring slowly over many cycles of the reference signal. This technique provides accurate and stable synchronization between the reference and generated signals as long as the reference signal remains stable, and drift in phase and frequency of the input reference signal is minimized between reference cycles. This technique is used primarily where many cycles (i.e. up to hundreds) of the generated signal must occur between phase reference signals without the relationship of the generated signal to the reference signal becoming indeterminate. Long term drift in the input reference signal can be accurately followed in the generated signal. Rapid or nearly instantaneous changes in the reference will, however, require the occurrence of several cycles of the reference signal before close synchronizaton between the reference and the internally generated signals is again achieved.
Another known technique assumes that the reference signal will change frequently and quickly. This technique is useful when the frequency ratio of reference to generated signals is small. A system using this type of technique can track the reference signal changes and acquire synchronization quickly after a change but still requires several reference cycles to achieve close synchronization after such a change occurs. Such a system, however, cannot hold synchronization if the inputed reference signal is interrupted or distorted, because the system will attempt to track the reference signal distortion or will rapidly drift away from the desired phase relationship between the reference signal and the generated signal during an interruption of the reference signal.
In both of the aforementioned techniques, the frequency and duration of periods during which synchronization is lost determines how accurately information can be manipulated within the system and must be considered when utilizing synchronization techiques to extract information from or insert information into a signal. An additional consideration is the amount of data that can be transferred when the synchronizing reference signal and information are contained in the same signal. The portion of the signal containing reference information subtracts from the total amount of useful information that can be conveyed in the signal. Known techniques either use a large amount (up to 50%) of the signal to contain reference information to obtain the best possible synchronization or use a lesser portion of the signal for reference information and assume that a stability of the signal will assure accurate information manipulation.
In cases where the inclusion of externally generated input reference information throughout the signal is not practical and long term stability of the signal is uncertain, only compromise techniques are currently available to attempt achieving the desired synchronization of signals.
From the foregoing considerations, it should be apparent that there is a great need for apparatus which improves rapid synchronization between an internally generated signal and a variable external signal. It is, therefore, an object of the invention to provide apparatus with the improved signal synchronization.
Another object of the invention is to allow rapid, accurate, and stable phase synchronization of an external reference signal to an internally generated signal even when the nature of the reference signal changes rapidly and drastically.
A further object of the invention is to allow a single input reference cycle to contain all required synchronizing information, to allow synchronization to be achieved quickly requiring only one occurrence of the input phase reference signal and only a few cycles of the generated signal, and to allow the synchronization to be maintained even when the ratio of a generated signal frequency to the reference signal frequency is very large.
Other objects and features of the present invention will further become apparent hereinafter with reference to the accompanying drawings and detailed description of the invention.